Add to functions to force the page editor to use style.css:

functions.php

function add_theme_editor_styles() {
    // 1. Core opt-in support for block editor stylesheets
    add_theme_support( 'editor-styles' );

    // 2. Load the main style.css asset into the editor canvas iframe
    add_editor_style( 'style.css' );
}
add_action( 'after_setup_theme', 'add_theme_editor_styles' );